Developmental and environmental effects on VTC2-dependent leaf ascorbate accumulation and functions

In nature, environmental conditions strongly fluctuate, frequently subjecting plants to periods of immediate photo-oxidative stress. The small molecule ascorbate allows plants to cope with such stress conditions. Ascorbate scavenges reactive oxygen species and enables the rapid and full induction of photoprotective non-photochemical quenching (NPQ). NPQ is dependent on zeaxanthin (Zx), which requires ascorbate as the electron donor during its synthesis by the violaxanthin de-epoxidase. The VTC2 gene encodes for one of two isoforms of GDP-L-galactose phosphorylase, the rate-controlling enzyme of ascorbate biosynthesis. In the current study, by including a newly identified vtc2 allele, we found that loss of VTC2 depleted ascorbate mainly from the mature leaves and thereby limited NPQ specifically in this tissue. Growth in fluctuating light and controlled climate suppressed the slow NPQ induction phenotype of vtc2 mature leaves to some degree. This was concurrent with a constitutively higher accumulation of zeaxanthin under this condition. When plants were shifted to natural conditions, with strongly fluctuating light and temperature, the ascorbate deficient mature leaves of vtc2 bleached. Together, our results reveal developmental and environmental effects on VTC2-dependent ascorbate accumulation and function.